1. Field of the Invention
The present invention relates to a fuel injection system or, in particular, to a fuel injection system employed in a diesel engine.
2. Description of the Related Art
Generally, an accumulator (common-rail) fuel injection system has such a configuration that high-pressure fuel, supplied from a high-pressure chamber, is introduced into a control chamber arranged in a fuel injection valve and a nozzle body is lowered to maintain a fuel injection port in closed state. Further, the fuel in the control chamber is allowed to leak into fuel discharge paths so that the internal pressure of the control chamber is reduced thereby to raise the nozzle body, with the result that the fuel injection port is opened to inject the fuel (see, for example, Japanese Unexamined Patent Publications. Nos. 2003-021017, 11-022580, 11-022583 and 11-022584).
In order to inject a predetermined amount of fuel within a short time, the fuel in the control chamber must be discharged in one operation into the fuel discharge paths at the sacrifice of a pressure pulsation caused in the fuel discharge paths. Normally, a check valve is arranged in the discharge path downstream of each injector, and a pressure pulsation may be caused also by the operation of a check valve. A sustained pressure pulsation in the fuel discharge paths will change the force acting on the control valve, etc. and adversely affects the operation of the nozzle body, thereby changing the amount of fuel injected.
Especially in the case where the interval is short between one injection and another, the fuel may be injected during a large pressure pulsation caused, in the fuel discharge path, by the preceding injection. In such a case, if the time when the pressure in the fuel discharge path drops coincides with the injection timing of the injector, the force acting on the control valve is reduced and the control valve lifts at a higher rate, and the fuel is rapidly discharged from the control chamber. Thus, the nozzle body rises rapidly, often resulting in the injected fuel amount being increased beyond the desired amount. In the case where the time when the internal pressure of the fuel discharge path rises coincides with the injection timing of the injector, on the contrary, the amount of fuel injected is liable to be smaller than required.
Further, the recent trend toward the employment of a pilot injection before the main injection and a post injection after the main injection has further shortened the interval between injections. This increases the possibility of frequent changes or variations in the amount of fuel injected. Also, an increased pressure pulsation in each fuel discharge path is accompanied by frequent cavitation and promotes the erosion of the actuator chamber of the actuator for controlling the control valve, thereby leading to a shorter service life of the parts. Although the pressure pulsation is attenuated by a longer distance, of the fuel discharge path, between the injectors, the recent demand for a smaller fuel injection system makes it difficult to shorten the distance of each fuel discharge path between the injectors to such a degree as to completely attenuate the pressure pulsation.
This invention has been achieved in view of this situation, and the object thereof is to provide a fuel injection system, for an internal combustion engine, in which the pressure pulsation in the fuel discharge paths from the injectors is reduced while, at the same time, suppressing the variations in the amount of fuel injected from each injector.